Gaseous hydrocarbons such as natural gas, LPG or LNG may contain small amounts of free oxygen, i.e. O2 gas. Free oxygen may be introduced inadvertently, by use of a gaseous hydrocarbon as a stripper gas or by blending with air. For example, natural gas may contain free oxygen as a result of poor purging after maintenance, air leakage into stripper pumps, use of natural gas as stripper gas for gas dryers, use of natural gas as stripper gas for water injection and from dissolved air in fluids injected down hole. The amount of free oxygen in the natural gas recovered from these processes may be in the range 70 to 100 ppm (vol). Alternatively, free oxygen may be introduced into LPG or LNG by blending processes with air to reduce calorific value in so-called “air balancing”. The amount of free oxygen introduced into LPG or LNG in this way may be as much as 0.5% vol.
The presence of free oxygen is potentially hazardous although the main concern in processing gaseous hydrocarbons containing free oxygen is corrosion to process equipment, resulting in costly replacement and maintenance. It is therefore desirable to limit free oxygen content to a few ppm or less.
Direct combustion of the free oxygen by heating the gaseous hydrocarbon over a combustion catalyst requires temperatures of 300° C. or more and it is not practical to heat large volumes of gas to this temperature and then cool it for subsequent use.
We have devised a process that overcomes these problems.